Typically the top of a box is closed by four flaps. Three of the flaps are positioned beneath the fourth or overlying flap.
When at least one of the flaps is made of plastic material or wax covered wherein a heated seal is contemplated between the plastics, the wax, wax to paperboard, or plastic to paper board, hot gases are broadcast onto the top of the underlying or bottom flaps and onto the bottom of the overlying flap. The overlying flap is then pressed downward onto the lower flaps to complete the plastics to plastics, wax to wax, plastics to board, or wax to board seal.
The hot gases directed to the underlying flaps cause the flaps to droop. The droop is caused by the expansion of the plastic material or wax on the top surface of the underlying flaps. When the overlying flap is pressed downward, it does not contact sufficient area of the underlying flaps to cause a good seal because the droop has moved the lower flaps away from the overlying flap. The seal, then, is merely at the flap hinge lines along the boundary of the box.